1. Field of the Invention
The present invention relates to a method for perforating a heat-sensitive stencil sheet, and more specifically relates to a method of perforating a heat-sensitive stencil sheet by exposing it to a visible or infrared ray to make a master for stencil or screen printing, and a heat-sensitive stencil sheet and a composition useful for the method.
2. Description of Related Art
As a structure of conventional heat-sensitive stencil sheets, is known a multilayer which is composed of a thermoplastic film laminated to an ink-permeable porous substrate made of Japanese paper or the like, or one layer which is composed simply of a thermoplastic film.
Methods for perforating such heat-sensitive stencil sheets to obtain masters for stencil or screen printing, include (1) a process of overlaying a heat-sensitive stencil sheet on images or letters that have been formed with carbon-containing materials such as pencils and toner by hand-writing or photocopying, and then exposing it to light from flash lamps, infrared lamps or the like to cause the portions of letters or images to emit heat so that the thermoplastic film of the stencil sheet is molten and perforated at portions that contact the images or letters, and (2) a process of melting and perforating the thermoplastic film of the stencil sheet by bringing the stencil sheet into contact with a thermal printing head which emits heat in dot-matrix forms so as to reproduce images in accordance with image data of electric signals that original images or letters have been transformed into.
In the above process (1), however, failure in perforation often occurs due to insufficient contact of the thermoplastic film of the stencil sheet with the original or the photocopied image portions of toner from which heat is emitted, or problems on so-called "pin holes" also occur which are phenomena of perforations caused in the stencil sheet at undesired portions by heat emitted from dust on the surface of the original or toner scattered out of the image portions. In the above process (2), there often occur perforation failure, conveying failure and wrinkling of the stencil sheet due to unevenness of pressure exerted to press the stencil sheet to the thermal printing head.
In order to solve such problems, the present inventor suggested, in Japanese Patent Application No. 284610/95, a method for perforating a heat-sensitive stencil sheet, which comprises ejecting a photothermal conversion material contained in a liquid from a liquid-ejecting means to transfer it together with said liquid to a heat-sensitive stencil sheet, and then exposing said heat-sensitive stencil sheet to a visible or infrared ray to perforate said heat-sensitive stencil sheet specifically at portions to which said photothermal conversion material has been transferred. This perforating method comprises a first step of transferring a photothermal conversion material to a heat-sensitive stencil sheet by ejecting a liquid, which contains the photothermal conversion material, to the heat-sensitive stencil sheet from a liquid-ejecting means which is out of contact with the stencil sheet, and the second step of perforating the heat-sensitive stencil sheet specifically at sites to which the photothermal conversion material has been transferred, by subjecting the stencil sheet to a visible or infrared ray.
The perforation method is advantageous in that little pin hole is formed in the stencil sheet since the stencil sheet does not have to be brought into contact with the original or the liquid ejecting means upon perforation. Similarly, since the stencil sheet is liberated from contact with the original or a thermal printing head that has been required in conventional perforating methods, any problem of perforation failure due to contact failure does not occur, and the stencil sheet is perforated faithfully to image information.
The present inventor also suggested, in the above Japanese Patent Application No. 284610/95, that a liquid absorbing layer is provided with a heat-sensitive stencil sheet on a surface to which the liquid is to be ejected, in order to prevent the liquid from spreading on the surface of the stencil sheet and promote the liquid to dry. In this perforating method, however, quality of perforations in stencil sheets is often greatly influenced by a condition of the liquid transferred to the liquid absorbing layer. In other words, if the liquid transferred to the liquid absorbing layer blots or spreads larger than the size of droplets of the liquid ejected from a liquid ejecting means, and then is exposed to a visible or infrared ray, perforations are also made larger in size, through which a large amount of ink are passed upon printing, yielding a blurred and unclear image on prints. Conversely, if the liquid does not have sufficient affinity with the liquid absorbing layer, the liquid is repelled by the liquid absorbing layer and causes so-called beading phenomena on the layer. In this case, the liquid is difficult to be fixed to the absorbing layer, and takes much time to dry. If a visible or infrared ray is radiated to the liquid absorbing layer in that state, much energy and time are required to perforate the stencil sheet, and perforations which form an image or letter are not uniformly made, yielding unclear and too light images on prints.
It is an object of the present invention to provide a method of perforating a heat-sensitive stencil sheet, which overcomes the above mentioned problems, and in which a liquid ejected from the liquid ejecting means and transferred onto the liquid absorbing layer is fixed faithfully thereto to provide a clear image. It is another object of the present invention to provide a heat-sensitive stencil sheet and a composition which are useful in the above method of perforating a heat-sensitive stencil sheet.